|Publication number||US5126870 A|
|Application number||US 07/455,104|
|Publication date||Jun 30, 1992|
|Filing date||Dec 22, 1989|
|Priority date||Dec 22, 1989|
|Also published as||WO1991010297A1|
|Publication number||07455104, 455104, US 5126870 A, US 5126870A, US-A-5126870, US5126870 A, US5126870A|
|Inventors||Richard M. Murphy, Peter W. Cornish, Gregory S. Maurer|
|Original Assignee||Raynet Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (11), Referenced by (3), Classifications (13), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to an intensity modulated optical signal generator which is capable of producing a linearized broadband optical signal, as required for frequency division multiplexed (FDM) amplitude modulated (AM) video.
With the advent of optical fiber technology wherein single mode optical fibers are now commonly available for transmitting information, a need exists for high bandwidth optical signal generators. Numerous high bandwidth optical signal generators have been proposed, however, each has its own drawbacks. Specifically, distributed feedback lasers are available for generating high bandwidth optical signals, but a disadvantage of such lasers is that the resulting high bandwidth signal has an undue amount of distortion and an undesirably low optical power output, especially when used for generating AM modulated signals.
It has also been proposed to generate a high bandwidth optical signal by utilizing a constant output laser and modulating an output thereof utilizing an external modulator, preferably a Mach-Zehnder interferometer. This technique also results in substantial signal output nonlinearities which are received as distortion. Johnson et al., U.S. application Serial Nos. 07/343,039; 07/412,656; and "Reduction of Intermodulation Distortion in Interferomic Optical Modulators" Optic Letters, Vol. 13, No. 10, October 1988, the disclosures of which are all incorporated herein by reference, disclose a method for substantially reducing nonlinearities attributable to third order products by adjusting relative amounts of the transverse electric (TE) and transverse magnetic (TM) power of an optical signal propagating through the external modulator to cancel the cubic dependence of the output optical power on drive voltage. More specifically, Johnson et al. propose to control the ratio of the power in the TE and TM modes by adjusting the polarization angle of polarized light at the optical input to the modulator such that the third order product of the modulated TE mode substantially cancels the third order product of the modulated TM mode. A preferred implementation of this technique is for use with a Mach-Zehnder interferometer.
Polarization control of a single external modulator to reduce third order products has two significant drawbacks. First, a frequency response of external modulators, in particular Mach-Zehnder modulators fabricated in lithium niobate (L1NbO3), is significantly different in the TE and TM modes. This effect ultimately limits the performance of the linearized modulator for broadband applications and is most significant below 200 MHz. Second, minimum second order distortion is only achieved when inflection points of the TE and TM transfer curves are precisely coincident and when the DC bias voltage is aligned to these inflection points. Unless the input polarization is precisely controlled electronically, the bias voltage must be used to achieve third order cancellation. However, since the bias control is optimally set to minimize second order products, using it to minimize third order products results in a direct tradeoff between second and third order distortion. Both cannot be optimized simultaneously with an external modulator unless the modulator is specially fabricated to very rigid specifications. In applications that demand very low distortion the single modulator implementation is most useful only for sub-optic applications where the second order distortion components fall out of band and can be filtered out electronically in a receiver.
It is an object of the present invention to provide a method and means for generating a modulated optical signal which has lower distortion than heretofore proposed.
It is a further object of the invention to provide a method and means for generating a modulated optical signal wherein third order modulation products are substantially eliminated as are all even order products.
This and other objects are achieved by a two modulator design that relies on a third order distortion cancellation technique that avoids the limitations of the polarization control single modulator approach.
According to one preferred embodiment, an optical carrier wave having a relatively constant amplitude is split into two branches by a non-symmetric optical power splitter, with the two branches being connected to primary and secondary external modulators, with most of the optical power propagating through one of the modulators, e.g. the primary modulator. The low power output branch is applied to the secondary modulator whose output is then recombined with a modulator output from the primary modulator at an output power combiner. Preferably, both modulators are identical in design, and bias voltages for each of the modulators are adjusted at inflection points of each modulator to minimize even order products. The bias points are also set so as to produce opposite polarity of modulation so as to allow for third order product cancellation, or alternatively any higher odd order product cancellation, as desired. Each modulator is modulated using a common RF electrical signal source, and an electrical attenuator is disposed between the RF signal generator and an input to the primary modulator to set a relative gain difference between the two modulators. According to this approach, by appropriately setting the value of the electrical attenuator, outputs from the primary and secondary modulators can be adjusted such that when their optical outputs are combined, third order products of one of the modulated outputs cancels third order products of the other modulated output.
According to an alternative preferred embodiment, instead of using a set electrical attenuator, a passive non-exact electrical attenuator in combination with a mode extinction modulator disposed downstream of an output of one of the modulators prior to this output being combined with an output from the primary modulator could be used.
Further embodiments also include a phase modulator in the path of one of the modulator outputs so as to optimize a total power output from the combined modulated optical signals, e.g. the first order product, by keeping the phase of the optical carriers at the optical combiner aligned.
According to yet a further embodiment, rather than using a single light source and the optical splitter as heretofore mentioned, it is also possible to provide separate incoherent light sources to drive the primary and secondary modulators which has the advantage of eliminating any kind of feedback loop for keeping outputs of the primary and secondary modulators optically in phase.
Preferaboly, the invention is implementable on a single integrated optical circuit. Titanium diffused lithium niobate is one preferred choice. Alternative modulators and/or manufacturing techniques are of course useable with the invention, for example proton exchange techniques on a lithium niobate substrate which yield high rejection of TM propagation modes.
These and other objects of the invention will be further explained by reference to the accompanying drawings and detailed description.
FIG. 1 illustrates a first preferred embodiment of the invention whereby an electrical attenuator is precisely adjusted for reducing intermodulation distortion;
FIG. 2 illustrates an alternate embodiment of the invention whereby a passive electrical attenuator is used in combination with a mode extinction modulator which has a bias control which is alternatively adjusted for reducing intermodulation distortion;
FIGS. 3a, 3b illustrate transform curves representative of outputs of the primary and secondary modulators; and
FIG. 4 illustrates a further embodiment of the invention which utilizes feedback control.
FIG. 1 illustrates a first preferred embodiment of the invention. Referring to this figure, an optical signal generator 1 transmits an optical carrier wave signal Pin along path 2, e.g. a wave guide or air, the optical signal preferably having a relatively constant amplitude. Pin is split into primary and secondary input paths 3, 4 via optical splitter 5 and coupled into primary and secondary modulators 6, 7 respectively. A radio frequency electrical signal generator 9 generates a modulated radio frequency electrical signal Vin (t) and transmits the signal through electrical attenuator 10 which is then summed at 11 with primary voltage bias Vb 1 disposed in series with Vin (t). The summed signal is then transmitted to the primary modulator 6 so as to modulate its input Pin1. Vin (t) is also transmitted through a secondary DC voltage bias control Vb2 and inputted into the secondary modulator 7 so as to modulate its input Pin. Outputs from the primary and secondary modulators P1 (t) and P2 (t) are combined by an optical combiner 24 so as to form Pout (t). Pout (t) is then transmitted to a remote detector 8 via any suitable means, e.g. single or multimode optical fiber, where it is converted into an electrical signal and optimally amplified by amplifier 9.
FIG. 2 illustrates another preferred embodiment of the invention wherein like components as described by reference to FIG. 1 have been similarly identified. In the embodiment of FIG. 2, the attenuator 10 of FIG. 1 has been replaced by a less precisely set attenuator 25, and an active mode extinction modulator 31 having a DC voltage bias control Vb3. Third order cancellation is achieved by precisely adjusting Vb3.
According to the invention, distortion cancellation is achieved as follows. The output of the modulators, P1 (t) and P2 (t) can be expressed as P(t), ##EQU1## where θ is proportional to a constant phase bias Vb ; and Pin is the input optical power (see FIG. 3a). The time varying phase modulation φ(t) is proportional to the modulator drive voltage Vin (t).
For the primary modulator, 6 (FIG. 1), a phase bias voltage Vb1 is applied, corresponding to θ1 =-π/2. This is an inflection point on the Cosine curve, and modulation about this bias location will produce no even order distortion. Vb2 is set to provide θ2 =π/2 in the secondary modulator 7, also a point where no even order distortions are produced, but opposite in sign from θ1 providing modulation sense in opposition to that produced in the primary modulator 6. Optical input power Pin1 is applied to 6, and optical input power Pin2 is applied to 7. Other bias points satisfying θ=π/2 ±nπ, n being an integer, may be used so long as the modulation senses of the two modulators are opposite of each other (FIG. 3b). The optical outputs for the two modulators can then be written: ##EQU2## The Sine functions can be series expanded, resulting in ##EQU3## where the higher order terms, resulting in indiscernible distortions in real systems, have been truncated for this discussion.
The modulators have a sensitivity of voltage applied Vin (t) to phase φ(t). This is usually defined in terms of the voltage required to change φ by π radians, and is referred to as Vπ. The substitution x=Vin (t) will be made for brevity in the expressions to follow. At the input to the primary modulator, applied voltage x is passed through an attenuator 10 having a voltage attenuation of γ. This corresponds to 20 log[γ] dB attenuation. The optical outputs of the two modulators can then be written as a function of their respective optical input power Pin, applied voltage x, voltage to gain sensitivities Vπ, and for the case of the primary modulator, electrical input attenuation constant γ. ##EQU4##
According to one embodiment, the optical splitter 5 is designed to have a power ratio between the two output ports of γ3, such that ##EQU5## This results in ##EQU6##
The phase modulators 6 and 7 provided with a DC voltage to set them at a fixed phase allows the phase of the optical carriers P1 (t) and P2 (t) to have zero offset as they are summed in optical combiner, 24. The output of this combiner has Pout (t)=P1 (t)=P2 (t). As should be apparent by examination of the equations for P1 (t) and P2 (t), the coefficient for the cubic term of x is zero in the sum, ##EQU7##
In a practical embodiment, the selection of γ3 may begin with the ratio of optical power in the paths from the primary and secondary modulators. From this, the value of the required electrical attenuator may be calculated.
Where there may be a difference α in modulation sensitivities Vπ between the primary and secondary modulators, such that αVπprim = Vπsec, the relationship between the optical power in the modulator paths and the electrical attenuation γ may be modified such that the ratio in optical power equals (αγ)3
Alternative embodiments allow the required optical power ratio γ3 to be produced by means than by the optical splitter 5. The required optical ratio may be produced by an unbalanced optical combiner 24 or by the use of a mode extinction modulator 31 as in FIG. 2.
Precise setting of the attenuator will allow complete third order cancellation. The primary and secondary DC voltage bias controls Vb1 and Vb2 are adjusted so as to produce operation of the modulators at specific bias points, i.e. inflection points on their respective transfer curves with opposite modulation sensitivities. Since the modulation sense is opposite it is possible to set the attenuator 10 so as to minimize any desired odd order modulation product, optimally third order products. Also, since operation is around the inflection points, all even ordered distortions can be eliminated as well. The exact setting will allow modulation with no even order products.
According to a further preferred embodiment, the combined modulated carrier wave output Pout (t) can be analyzed using an analyzer 51 (FIG. 4). Active feedback control can then be utilized so as to precisely minimize any desired output modulation distortion product. Specifically, the analyzer could be set to analyze even ordered modulation products and hence actively control the phase bias voltages Vb1 and Vb2 if desired so as to insure that these products are maintained at a minimum, preferably 0. In addition, the analyzer could also analyze third ordered products and actively control either the magnitude of the attenuation of the electrical signal inputted to the primary modulator or the mode extinction modulator of FIG. 2, for example. Alternate embodiments would include active power splitters 5 and active power combiner 24 which could be controlled as well if desired, all such embodiments being included within the invention.
According to a preferred embodiment, the modulator design is preferably precisely implemented on a single integrated optical circuit. In addition, a further embodiment would be the design of a circuit such that optimum distortion cancellation is achieved without any active control so as to simplify product design, though active control using the analyzer 51 could be utilized if desired.
Though the invention has been described by reference to certain preferred embodiments thereof the invention is not to be so limited and is only to be limited by the accompanying claims.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US4012113 *||Dec 17, 1975||Mar 15, 1977||Herwig Werner Kogelnik||Adjustable optical switch or modulator|
|US4127320 *||Jun 29, 1977||Nov 28, 1978||Bell Telephone Laboratories, Incorporated||Multimode optical modulator/switch|
|US4291939 *||Mar 24, 1978||Sep 29, 1981||The United States Of America As Represented By The Secretary Of The Navy||Polarization-independent optical switches/modulators|
|US4340272 *||Apr 2, 1980||Jul 20, 1982||Thomson-Csf||Light intensity modulator in an integrated optical circuit with feedback means|
|US4691984 *||Sep 26, 1985||Sep 8, 1987||Trw Inc.||Wavelength-independent polarization converter|
|US4711515 *||May 29, 1984||Dec 8, 1987||American Telephone And Telegraph Company, At&T Bell Laboratories||Electrooptic polarization multiplexer/demultiplexer|
|US4752120 *||Mar 18, 1986||Jun 21, 1988||Nec Corporation||Polarization controlling device comprising a beam splitter for controllably bifurcating an input polarized beam to two polarization controlling elements|
|US4769853 *||Oct 23, 1987||Sep 6, 1988||Trw Inc.||High dynamic range fiber optical link|
|US4798434 *||Sep 9, 1987||Jan 17, 1989||U.S. Philips Corp.||Optical polarization regulator having a wave-guide structure|
|US4820009 *||Aug 13, 1987||Apr 11, 1989||Trw Inc.||Electrooptical switch and modulator|
|US5031235 *||Oct 27, 1989||Jul 9, 1991||Hoechst Celanese Corp.||Cable system incorporating highly linear optical modulator|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US5394261 *||Nov 19, 1993||Feb 28, 1995||Canon Kabushiki Kaisha||Optical communication system and transmitting apparatus for use therein|
|US6753993 *||May 30, 2003||Jun 22, 2004||Lucent Technologies Inc.||Passive optical wavelength converter|
|US20090009259 *||Jan 12, 2007||Jan 8, 2009||Tomoaki Ohira||Angle modulator|
|U.S. Classification||398/79, 398/186, 398/194|
|International Classification||H04B10/155, G02F1/03|
|Cooperative Classification||G02F1/0327, H04B10/58, G02F2203/19, H04B10/5053|
|European Classification||H04B10/505, H04B10/58, H04B10/5053, G02F1/03D|
|Feb 9, 1990||AS||Assignment|
Owner name: RAYNET CORPORATION, CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNORS:MURPHY, RICHARD M.;CORNISH, PETER W.;MAURER, GREGORY S.;REEL/FRAME:005247/0384;SIGNING DATES FROM 19900102 TO 19900201
|Apr 3, 1995||AS||Assignment|
Owner name: ERICSSON RAYNET, A DE GENERAL PARTNERSHIP, CALIFOR
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:RAYNET CORPORATION, THROUGH MERGER WITH AND INTO RAYNET INTERNATIONAL, INC.;REEL/FRAME:007440/0633
Effective date: 19941116
|Dec 18, 1995||FPAY||Fee payment|
Year of fee payment: 4
|Dec 29, 1999||FPAY||Fee payment|
Year of fee payment: 8
|Dec 30, 2003||FPAY||Fee payment|
Year of fee payment: 12